Milk fat globules secretion

Gaining an understanding of the molecular mechanism of crescent formation would advance our understanding of the process of milk fat globule secretion. Crescents are a route by which cellular constituents can... 

PAS 6/7 has no known role in milk-fat globule secretion. The human homologue, lactadherin, binds to rotaviruses and may protect the gastrointestinal tract of the suckling young from infection (Peterson et al., 2001). 

Patton, S., Stemberger, B.FT, Knudson, C.M. 1977. The suppression of milk fat globule secretion by colchicine an effect coupled to inhibition of exocytosis. Biochim. Biophys. Acta 499, 404 110. 

Baumrucker, C. R. and Keenan, T. W. 1973. Membranes of mammary gland. VII. Stability of milk fat globule membrane in secreted milk. J. Dairy Sci. 56, 1092-1094. 

It was long speculated that BTN plays some role in milk fat secretion (Mather and Jack, 1993) and now there is direct evidence for this. Ogg et al. (2004) prepared two lines of mice in which either variable amounts of a truncated form of BTN were expressed, or BTN expression was undetectable. Females of both lines were fertile and entered into lactation but had defects in fat globule secretion. In these mice, just like in (Xdh+I ) mice, secretory cells became engorged with lipids. Lipid droplets were secreted with disrupted membranes and aggregated into large droplets. 

One major obstacle to molecular studies in the area of milk-fat globule formation and secretion is the lack of a cell line that secretes, or can be induced to secrete, milk fat globules, although some progress in this area has been reported (Rohlfs et al, 1993). Development of such a cell line would be a major advance, in and of itself, and would be an invaluable aid in fostering further research in this area. 

Heid, H.W., Keenan, T.W. 2005. Intracellular origin and secretion of milk fat globules. Eur. J. Cell Biol. 84, 245-258. 

Jack, L.J.W., Mather, I.H. 1990. Cloning and analysis of cDNA encoding bovine butyrophilin, an apical glycoprotein expressed in mammary tissue and secreted in association with the milk fat globule membrane during lactation. 1. Biol. Chem. 265, 14482-14486. 

In some milk fat globules, small aqueous compartments are located beneath the membrane bilayer, which have been termed cytoplasmic crescents (Huston and Patton, 1990). Whether or not this cytoplasmic inclusion provides some benefit is unknown. Yet, as this aqueous compartment is protected from the bulk serum phase by the MFGM, constituents located therein are presumably afforded some protection, at least initially, from gastric hydrolysis. Huston and Patton (1990) found crescents in all samples of milk they examined, and they were more prevalent in human (7.3% of globules), than in bovine (1% of globules) milk. Furthermore, there was considerable individual and diurnal variation. The structure of a cytoplasmic inclusion, surrounded by an intact plasma membrane on one side and a fat globule surface on the other, may allow certain labile constituents to be protected until they reach their proper site of bioactivity. At this point it is not known whether the crescents have a purpose or are simply the result of inefficiencies in the secretion process. As it is possible to isolate milk preparations enriched in cytoplasmic crescents, there is an opportunity to determine the nature of the materials found within. This unusual biocompartment may prove to be a model of food structure for biodelivery. 

Evers, J.M. 2004. The milk fat globule membrane - compositional and structural changes post secretion by the mammary secretory cell. Int. Dairy J. 14, 661-674. 

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